In deepwater ocean bottom nodes surveys, autonomous seismic detectors are placed on the seabed and make continuous or intermittent seismic recordings. During the recording periods, a plurality of seismic signals (“shots”) are transmitted into the ocean and/or into the seafloor for the purpose of seismically illuminating the subsurface formations, as illustrated schematically in FIG. 1. Seismic signals that have been refracted, reflected and/or transmitted through the subsurface are recorded by the seismic detectors. After recording the resulting seismic signals for a period of time, the nodes are recovered and the seismic data they have recorded are processed for the purpose of extracting information about the subsurface. In order to extract meaningful information, it is advantageous to determine the location and relative timing of each seismic node.
Typically, the placement of any given node on the seabed is known to only limited accuracy, due to limitations of the positioning and placement equipment. In addition, there may be operator errors in the positioning process, which add to the errors arising from equipment limitations.
Still further, the very precise clocks that are needed for seismic data analyses tend to drift unpredictably over time. Some clocks may lose time while others may gain it, and the rate of drift for a given clock may vary over time. Over the two month time period of a typical seismic survey, clocks may easily drift as much as 50 ms in either direction. Finally, operator errors in the initial clock synchronization or final data collection may add to the drift-derived errors.
Knowledge about the location of the seismic node may be improved by comparing the arrival times of many active shots from the sea surface to each node. This location will have limited accuracy, however, because of uncertainties in the location of the active shots and changes in water velocity over the time interval of the active shooting program.
For these reasons, there remains a need for improving the degree of precision that is possible in ocean bottom seismic surveys.